1. Field of the Invention
This invention relates to a color separating optical system, and more particularly to a color separating optical system for use in a color television camera.
2. Description of the Prior Art
The television camera presently having the widest usage in television broadcasting is the RBG system 3P camera employing three 11/4 inch or 1 inch plumbicon image pick-up tubes. As the color separating optical system, these cameras often employ an optical system constituted of a number of prisms and known as the Philips system after the Philips Company by which it was developed. In order to attain uniform sensitivity in these cameras, an F/2.2 objective is used as the standard lens for the 11/4 inch plumbicon and an F/1.6 objective is used as the standard lens for the 1 inch plumbicon.
In addition to the above-mentioned 11/4 inch and 1 inch image pick-up tubes, there has recently been developed a 2/3 inch tube which is coming into increasingly wider use along with the rise in demand for compact and handy cameras. Although it would be desirable to use an objective of a brightness as great as F/1.1 with such a 2/3 inch tube, this is not altogether feasible technically or geometrically. Still a lens of greater brightness than the F/1.6 lens used with the 1 inch plumbicon, say a lens on the order of F/1.4, is an absolute necessity in a camera employing a 2/3 inch plumbicon image pick-up tube.
There have also been other rapid developments in the field of television image pick-up tubes over the past few years and tubes other than the plumbicon such as the chalnicon and the saticon have also been applied to television broadcasting.
The plumbicon differs from the other two tubes mentioned in that its photoelectric conversion film is of high reflectivity so that a glass disc known as an anti-flare chip must be attached forward of the face plate in order to cut undesirable flare. The chalnicon, saticon and vidicon tubes do not require an anti-flare chip. Thus, in using a bright objective with the plumbicon image pick-up tube, it is possible to take full advantage of its performance only if careful consideration is given to the overall glass length, that is, not only to the glass length of the color separation prism which functions to compensate for the aberration of the objective but also to that of the face plate and anti-flare chip used in front of the image pick-up tube.
Strictly speaking, any objective regardless of its brightness is best used with the glass length envisioned at the time of design. As a practical matter, however, a lens of a brightness of less than around F/2 can be used fairly satisfactorily with a glass length differing somewhat from that initially envisioned. This is not true, however, in the case of a lens of a brightness on the order of F/1.4. Therefore, since lenses of different brightness are used with the plumbicon, saticon, chalnicon and other image pick-up tubes, it has not been possible to employ a common prism-type color separating optical system for all types of image pick-up tubes.
What is more, the sensitivity of the chalnicon tube increases more rapidly on the long wavelength side than does that of the plumbicon so that for this reason also it has been impossible to use a common color separating optical system.
However, as an increasing variety of image pick-up tubes come into practical use, the inconvenience of having to change the color separating optical system each time the image pick-up tube is changed will make itself more apparent. Thus, it is highly desirable to have a color separating system which can be easily adapted to match the requirements of any type of image pick-up tube.